Blender 2.81 hollow model for 3D printing
There are a few solutions that I use.

1) booleans - if the models is without errors like double faces etc. and the boolean works, just make union and then triangulate faces to avoid necessary ngons

2) I used external software meshmixer, its free and have some great functions for this purpose like solidify, which takes all the geometry and create just the "visible" part as the one solid object, then there is possibility to hollow it to make it like vase with certain wall thickness.

3) Voxel remesh - after blender introduced voxel remesh I stopped using meshmixer. You just join the object in blender to be one and then use voxel remesh which "bake" the object into one with no internal geometry.